Insight into the Advances in Clinical Trials of SARS-CoV-2 Vaccines

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has thrown a challenge to the scientific community. Several interventions to stop or limit the spread of infection have failed, and every time the virus emerges, it becomes more contagious and more deadly. Vaccinating a significant proportion of the population is one of the established methods to achieve herd immunity. More than 100 COVID-19 vaccines have been designed and tested against the virus. The development of a new vaccine takes years of testing, but due to the pandemic, healthcare authorities have given emergency use authorization for a few vaccines. Among them are BioNTech and Moderna vaccines (mRNA based); ChAdOx1, Gam-COVID-Vac, Janssen vaccines (vector-based); CoronaVac, COVAXIN (virus inactivated); and EpiVacCorona vaccine (viral peptide). Mixtures of vaccines are also being tested to evaluate their efficacy against mutant strains of SARS-CoV-2. All these vaccines in clinical trials have shown robust production of neutralizing antibodies sufficient to prevent infection. Some of the vaccinated people reported serious complications. However, no definitive relationship could be established between vaccination administration and the occurrence of these complications. None of the COVID-19 vaccines approved to date have been found to be effective against all of the SARS-CoV-2 variants.


Introduction
Human coronaviruses are respiratory viruses that were discovered in the 1960s, and seven strains have been identified to date [1]. Some human coronaviruses, like HCoV-229E, HCoV-OC43, HCoV-NL63, and HCoV-HKU1, are known to cause mild respiratory illnesses [2]. However, more infectious and dangerous strains such as severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East Respiratory Syndrome Coronavirus (MERS-CoV) emerged in 2002 and 2012, respectively [3]. e severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) was discovered in December 2019 and is the cause of coronavirus disease 2019 . It is related to SARS-CoV and MERS-CoV in terms of phylogeny [1,3]. e first case of COVID-19 was reported from Wuhan, China, in 2019. It soon spread to other parts of the world and was ultimately declared a pandemic in March 2020 by the World Health Organization (WHO). It has impacted hundreds of millions of people and claimed the lives of nearly three million [4]. It is a highly contagious disease, transmitted through respiratory droplets and direct contact with infected people. Also, the virus can cause a wide range of illnesses, from self-limited mild respiratory illness (80% of cases) to severe respiratory failure, multiple-organ failure, and death [5]. Elderly people, pregnant women, and people with underlying medical conditions are at higher risk of developing severe complications from COVID-19 [6].
With the urgent need to control the COVID-19 pandemic, the Food and Drug Administration (FDA) has created the Coronavirus Treatment Acceleration Program (CTAP), aiming to move new therapies as soon as possible to patients. A total of 490 trials have been reviewed by the FDA, and among them, 9 drugs have received emergency use authorization, and only one (Remdesivir) has been approved by the FDA for the treatment of COVID-19 in adults and paediatric patients aged more than 12 [7]. However, their efficacy was inconsistent in different study settings. In the absence of effective and safe therapeutic interventions for COVID-19, preventing the development of severe illness was considered one of the most suitable options [8].
Many studies and trials on vaccine development started immediately after the identification of the full genomic sequence of SARS-CoV-2. e studies to design a vaccine began in the early 2020s and are now progressing at a lightning pace. If, in recent times, a safe and effective COVID-19 vaccine is developed, then this could create history in modern medicine [9]. Several technological approaches have been adopted for developing the COVID-19 vaccines, and the most important ones are attenuated, protein, vector, and mRNA-based. Many vaccines have received emergency use authorization to administer the jab to the most vulnerable groups of the population. e Pfizer-BioNTech COVID-19 vaccine was the first COVID-19 vaccine to be authorized for emergency use by the FDA in December 2020. Subsequently, Moderna COVID-19 Vaccine and Janssen COVID-19 Vaccine have been authorized for emergency use by the FDA for the prevention of COVID-19. Moreover, other vaccines are in the authorization process [10].
With the aim of achieving herd immunity, countries have started a mass inoculation program with different types of COVID-19 vaccines. e safety and efficacy of vaccines depend to a large extent on the design and the process of development, as well as on the individual response shown by the host system. Hence, it is extremely essential to know all the effects that were observed during vaccine testing [9,10]. Several studies in the past have highlighted the important safety and efficacy parameters observed in this population in a clinical setting. Some vaccine trial data suggested increased chances of abortion and hemorrhagic complications in certain groups of participants [11]. is crucial information is essential for establishing the complete efficacy and safety of the vaccine but was not elaborately discussed in the previous studies [12]. erefore, the present study was planned with the purpose of compiling the critically important scientific data published by research organizations during the conduct of clinical trials and then analyzing it in a way that will help medical professionals and the public decide the most suitable vaccine for preventing COVID-19.

Methods
An online review of literature was conducted on PubMed, Google Scholar, and Science Direct websites using keywords such as "COVID-19," "Vaccine," "Clinical Data," "Trials," "Adverse Reaction," and "Mechanism." e review included clinical trials conducted from the beginning of 2020, coinciding with the reports of the successful design of the COVID-19 vaccine, until the end of July 2021 [9]. e review resulted in more than 3000 total articles. However, only 45 articles were selected for the present study based on the inclusion criteria. e authors independently reviewed the titles, abstracts, and text of the articles. e information such as English language, study center, number of subjects, study design, study protocol, dose, duration, route of administration, ethical approval, statistical methods, and biochemical estimations were considered the critical parameters for evaluating the content and were considered the inclusion criteria. Only those articles containing this information were selected for the analysis. Articles having this information were only included for further analysis [8].

Data Analysis
SARS-CoV-2 is an RNA virus that targets the angiotensin converting enzyme-2 of host cells, and this action occurs through specialized binding glycoproteins called spike proteins. is interaction is crucial for the progression of infection. e transmembrane serine protease of the host cell facilitates the entry of viruses. Inside the host cell, the RNA of the virus modulates the function of synthesizing different components, such as viral polyproteins, nucleic acids, and structural proteins [13]. Finally, these components are assembled and released to attack a new host cell. e steps involved in the life cycle of SARS-CoV-2 are considered vital targets to limit pathogenesis ( Figure 1). Almost all the vaccines designed and developed for COVID-19 are aimed at spiking proteins. Extensive research and testing for these vaccines began in early 2020 [14].
ere are more than 100 COVID-19 vaccines under various stages of development and clinical evaluation. ese vaccines can be classified as protein subunits, inactivated viruses, DNA-based, RNA-based, viral vectors, and liveattenuated vaccines (Figure 2). e United States, China, the European Union, the United Kingdom, and India are the top five countries that have done mass inoculation so far. Countries are using different types of vaccines to inoculate their population. Currently, no vaccine is certified to be superior/inferior in terms of safety and efficacy [14,15]. mRNA-based vaccines can be classified into two forms: nonreplicating mRNA and self-amplifying mRNA vaccines. e mRNAs are designed and synthesized in the laboratory. ey are incorporated into liposomes, so that the mRNA can be carried into the cell and prevented from degradation. Once inside the cell, the mRNA is translated into ribosomes to produce specific proteins (spike glycoproteins). e spike proteins are recognized by immune cells and stimulate antibody production [16].
DNA vaccines are also referred to as nucleic acid/genetic vaccines. ese vaccines contain the plasmid DND, derived from eukaryotes. After entering cells, the DNA is transcribed and translated to produce specific proteins. is stimulates the immunological system of the host to produce both specific and nonspecific responses, leading to the generation of antibodies [17]. e attenuated and viral component containing vaccines are designed in such a way that they trigger the immune cells to produce neutralizing antibodies [18]. e following sections summarize the important COVID-19 vaccines with a brief description of their characteristics. e analysis of the clinical trial data is represented in Tables 1-12. 3.1. BioNTech COVID-19 Vaccine. BioNTech has two vaccine candidates, such as BNT162b1 and BNT162b2 (Table 1). ese vaccines are based on mRNA technology and are derived with the modification of nucleosides and formulated in lipid. e mRNA codes for the receptorbinding domain of spike proteins. According to reports available, the serum IgG antibody concentration after the first dose was found to be comparable to the level observed in COVID-19 recovered patients [19]. Further, a dose-dependent response in the level of IgG antibodies was measured when 10 μg and 30 μg of the vaccine were tested in the study population. e elevation in the level of neutralizing antibodies was found to be 10X and 45X, respectively, for the two doses when compared to serum levels of COVID-19 patients. However, a further increase in the dose (100 μg) did not show any additional rise in serum IgG concentration [20]. e administration of BNT162b1 induced functional CD4 + and CD8 + in 95.2% of human volunteers. e CD4 + cells were found to target specifically the SARS-CoV-2 RBD [21]. A similar type of response (94.6%) was also observed when BNT162b2 was administered to study participants older than 16 years. After second dose administration, the immunity response showed a boost, especially in young and older adults, but in people between 65 and 85 years old, the immunological response was found to be weak. Many of the study members indicated manageable common adverse reactions, including a grade 3 decrease in lymphocyte count and grade 2 neutropenia [22]. A few serious adverse events such as atherosclerosis, cardiac arrest, and paroxysmal ventricular arrhythmia resulting in death were reported. However, cardiovascular and thrombotic events were also observed in placebos due to unknown causes [23].

CoronaVac COVID-19 Vaccine.
is vaccine was developed by a Chinese pharmaceutical firm called Sinovac Life Sciences. e inactivated strains of SARS-CoV-2 were created and purified from Vero cell lines and are used in the vaccine production [24]. Two doses of the vaccine (3 μg and 6 μg) were tested. e lower dose (3 μg) produced 88% of seroconversion rate, while the higher dose (6 μg) indicated 100% seroconversion rate. e two-dose vaccine needs to be administered at an interval of 14 days [25]. On 28th day of vaccination, both the doses (3 and 6 μg) stimulated the production of neutralizing antibodies but the higher dose (6 μg) of vaccine showed better immunogenic response [26]. e vaccine administration did not show any serious adverse reaction except in one case, where, within 48 hours of first shot, a volunteer experienced hypersensitive reaction such as urticaria [27]. e phase III analysis suggested that the vaccine administration produced 50% protective efficacy in preventing symptomatic infection, 78% in preventing mild cases requiring treatment and 100% in preventing severe form of infection (Table 2) [28].

ChAdOx1 nCoV-19 Vaccine.
is is a vector-based vaccine, designed and developed by Oxford University (Table 3). e genetic sequence for the full-length structural glycoprotein of SARS-CoV-2 with tissue plasminogen is  Sixty nursing home residents (NHR) (median age 87.5) were recruited, 18 of whom had never been infected with SARS-CoV-2. SARS-CoV-2-S targeting antibody and functional Tcell responses were the major outcomes.
In convalescent NHR, plasma antibody levels and SARS-CoV-2 S-reactive IFN-c CD8+ and/or CD 4+   On day 0, 14 schedule vaccine administration 6 mcg showed higher incidence of adverse effects compared to 3 mcg of vaccine. While on n day 0, and day 28 schedules, 3 mcg, 6 mcg, and placebo had no statistical difference in adverse effects. All adverse reactions were mild to moderate intensity. In phase 1, 144 participants (≥18 years old) were randomly assigned to 2 cohort day 0, 14 and 0, 28 schedules; within each cohort people were assigned for block 1 (3 mcg of vaccine or placebo) or block 2 (received either 6 mcg of vaccine or placebo). While in phase 2 study 600 participants were randomly assigned in two cohorts (0, 14 and 0, 28) and randomly assigned (2 : 2 : 1) to receive either 3 mcg, 6 mcg, or placebo In phase 1 trial, the seroconversion rate of three arms (3 mcg, 6 mcg, and placebo) was 46%, 50%, 0%, respectively, in days 0, 14 schedule, while it was higher in days 0, 28 schedule of 83%, 79%, 4%, respectively In phase 2 trials, it was seen for 92%, 98%, 3% respectively in 3 mcg, 6 mcg, and placebo arms in days, 0,14 schedule, whereas 0, 28 days schedule had a seroconversion rate of 97%, 100%, 0% among the three arms respectively In phase 1 trial, the adverse reactions occurrence was higher in 6 mcg vaccine arm compared to 3 mcg, and placebo in the days 0, 14 cohort, while 0, 28 cohort reported lower incidence of adverse reaction in the vaccine arms 3 mcg, 6 mcg vaccine, or placebo of 13%, 17%, 13%, respectively In phase 2 study, the adverse reaction occurrence in 3 mcg, 6 mcg, and placebo was 33%, 35%, 22%, respectively in days 0,14 cohort. While days 0, 28 cohort reported lower incidence of adverse reactions of 19%, 19%, 18%, respectively A total of 175 adverse events were reported; 3 of them were considered related to the intervention (vaccine or placebo); one case was in vaccine arm, one in placebo arm, and one case who remained masked to group allocation rombosis and thrombocytopenia after ChAdOx1 nCoV-19 vaccination [30] Case report A case report of 5 healthcare workers who received ChAdOx1 nCoV-19 vaccination -7-10 days after receiving first dose of CHADOX1, high levels of antibodies to platelet factor 4-polyanion complexes were documented in all patients without any previous exposure to heparin incorporated into a nonreplicating simian adenovirus vector called ChadOx1. After administration, the codons express the genes for the synthesis of spike protein by host cells. ese glycoproteins have antigenic properties and stimulate the production of antibodies. e first dose of the vaccine required 28 days to show peak antibody levels in the serum and was found to remain for 56 days [47]. e data from the clinical studies suggested that the vaccine is better tolerated by older adults. e second dose produced a better serological response in terms of elevated antibody levels and was found to be independent of participants' age. Vaccine efficacy was found to be high in volunteers receiving a low dose initially followed by a standard second dose. e vaccine in the study participants In comparison to placebo, the vaccine group reported more solicited injection site reactions after the first dose and the second dose and in younger adults than older adults Young participants (8%) had more adverse events than older participants (4%) and young participants who got a lower vaccine dosage had more adverse events than older participants (4%)  In both trials, the majority of participants had mild to moderate adverse effects A total of 7 participants had severe adverse effects but none of them were vaccine related  While the seroconversion to live SARS-CoV-2 virus was detected in 59% of the 1 × 10 11 viral particles group and 47% of the 5 × 10 10 viral particles group Fatigue, headache, and fever were the most often reported side effects. While the pain was the most common local adverse response produced nonserious adverse reactions. A few cases of hemolytic anemia and transverse myelitis were reported in vaccinated people, and the independent expert committee ruled out any direct relationship with the vaccine [17,29]. romboembolic events observed in AZD1222 vaccinated individuals have been extensively studied. e reports, after analyzing all the data, suggested that, in most of the patients who showed this adverse event, the presence of anti-platelet

Study name
Study type Trial design Efficacy Safety and adverse drug reactions Gam-COVID-Vac is a combined vector vaccine carrying full gene for SARS-CoV-2 glycoprotein S based on rAd type 26 (rAd26) and rAd type 5 (rAd5). Phase 1/2 trial showed a well-tolerated and high immunogenicity of the vaccine in healthy adults [41] Safety and efficacy of a rAd26 and rAd5 vector-based heterologous prime-boost COVID-19 vaccine: an interim analysis of a randomized controlled phase 3 trial in Russia [42] Phase 3 randomized controlled trial 21977 participants (>18 years old) were randomly assigned (3 : 1) to receive either vaccine (n � 16501) or placebo (n � 5476); 19866 of them received two doses of either vaccine or placebo with 21 days apart 21 days after the first vaccination, Gam-COVID-Vac showed an efficacy of 91%. Interestingly, vaccine efficacy was 91.8% in elderly participants while it was more than 78% in all ages.
Headache, injection-site reaction, and asthenia were the most common recorded symptoms. None of the serious adverse events were related to COVID-19 vaccine.  Table 12: Summary of common side effects and rare side effects associated with COVID-19 vaccines.

Moderna mRNA Vaccine.
is pharmaceutical company's COVID-19 vaccine is also based on mRNA technology (Table 4). e mRNA was designed to encode for S-2P antigens, which are SARS-CoV-2 glycoproteins having a transmembrane and an S1-S2 cleavage site. After vaccination, the host immune system was found to identify the antigens and produce IgG antibodies with a seroconversion rate of 100% by day 15 [48]. A dose-dependent enhancement in the IgG antibodies was observed in the study participants.
ree doses of the vaccine (25, 50, and 100 μg) were tested. In the phase-I clinical trials, 25 μg and 50 μg were tested, while, in phase-III, a higher dose (100 μg) was administered. Both the combinations of doses, such as 25 μg + 50 μg and 25 μg + 100 μg, produced a dose-dependent increase in the CD4+ involving 1 helper T cells. e phase III studies indicated that the level of protection against COVID-19 was 94.1%. e vaccine showed consistency in the protective action irrespective of the age (18-65 years and ≥65 years), sex, and ethnicity of participating members. e adverse reactions recorded for the different doses of vaccine were found to be the same as those observed with placebo and after any vaccination [31]. (Table 5). e strains of 19nCoV-CDC-Tan-HBO 2 were inactivated and purified by passing through Vero cell lines. Mass production of the vaccine was done in basket reactor, and a novel carrier was used to deliver the genetic sequence in the host cells. ree doses of the vaccine, namely, 2, 4, and 8 μg, were tested [49]. All the doses of vaccine produced higher seroconversion on day 28, while medium dose (4 μg) produced this effect on day 21 and highest dose (8 μg) on day 14. Further, not-much variation was observed in the levels of antibodies between medium (4 μg) and highest (8 μg) tested dose of vaccine on day 28. e serological analysis also indicated the higher concentrations of neutralizing antibodies mostly in younger adults compared to older adults. All the participants involved in vaccine testing reported mild side effects such as fever (>38.5 C) [32].

BBV152 Vaccine.
e vaccine was developed by an Indian company (Bharath Biotech) ( Table 6). e vaccine contains a whole virion-β-propiolactone-inactivated SARS-CoV-2. e strains of the virus are formulated in Algel molecules that assist in preventing the vaccine's degradation and entry into the host cells. Two doses of the vaccine, such as 3 μg and 6 μg, were tested. e dosage regimen was followed with a gap of 14 days between two doses. e neutralizing antibodies were found to have peaked on day 56 [33]. Antibodies against spike (S1) proteins, RBD and nucleocapsid proteins of SARS-CoV-2 such as CD4+, CD27+, 1, and 2 dependent antibody isotopes were present in the study participants.
e seroconversion rates of neutralizing antibodies for 3 μg and 6 μg were found to be 92.9% and 98.3%, respectively. All the members of the study reported mild, tolerable side effects (Grade 2/3) and none experienced any serious complications [34].

RBD-Based Protein Subunit Vaccine.
A Chinese biotechnological firm has designed and developed the vaccine, especially against the variants of SARS-CoV-2 (Table 7). e spike proteins' RBD dimer was used as the target after carefully analyzing the sequence of the mutated strains of the virus. e vaccine needs to be administered in three doses. e serum analysis indicated the level of neutralizing antibodies increased by 1.6-2.8-fold [35]. However, we are still awaiting complete data on the efficacy of the vaccine against the variants of SARS-CoV-2. Also, studies to confirm the type of neutralizing antibodies and their extent of seroconversion rate are in progress. e vaccine tested in different phases of clinical trials did not show major adverse reactions in the study participants [36].

EpiVacCorona Vaccine.
is vaccine is developed by a Russian Biological Research Center (Vector Institute) ( Table 8). A synthetic viral peptide was prepared that resembles the SARS-CoV-2 component. e administration of this component is reported to trigger the antigenic response in the body, stimulating the production of antibodies [37]. e vaccine is being tested on the population over 18 years of age.
e data from clinical trials indicated that the administration of two doses of vaccine activated the production of antibodies. We are still awaiting more details about the efficacy and safety of the vaccine [38].

Nonreplicating Adenovirus Type-5 (Ad5) Vectored COVID-19 Vaccine.
is vaccine was designed and developed based on the Admax system. A nonreplicating adenovirus (type-5) was used as a vector to carry the genetic information for expressing the SARS-CoV-2 spike proteins (Table 9). A cloning process was adopted to duplicate the genetic sequence of the 'S' proteins, which was then incorporated into Ad-5 along with the tissue plasminogen activator signal peptide gene. Postvaccination analysis in the healthy volunteers indicated the presence of a high concentration of neutralizing antibodies such as CD4 + and CD8 + . ese antibodies were found to be expressed by TNFα [39]. e levels of these antibodies were found to be dosedependently varied and were very high upon comparison with the placebo group. e documented adverse events suggest that all the participants well tolerated the side effects without showing any major complications. However, the efficacy of the vaccine in older people (>55 years) was observed to be low with lower antibody responses after two dose vaccinations [40].

Gam-COVID-Vac Vaccine.
e two recombination adenovirus vaccines are named rAD26-S and rAD5-S. e  COVID-19 vaccine response in pregnant and lactating women: a cohort study [57] Cohort study Participants in the U131 reproductive-age vaccine study were given either the Pfizer or the Modern vaccine. In comparison to nonpregnant women, the primary goal was to assess the immunogenicity and reactogenicity of the mRNA vaccination in pregnant and lactating women Vaccine induced antibodies titers were higher among pregnant and lactating compared to nonpregnant adults which were detected all in breastmilk and umbilical cord. e second vaccine dose showed a higher IgG titer but not IgA in maternal blood and breastmilk e vaccine-elicited immunoglobulin profile in milk after COVID-19 mRNAbased vaccination is IgGdominant and lacks secretory antibodies [58] Cohort study A total of 10 participants who received either Pfizer or Moderna vaccines were enrolled. e main aim was to assess the presence of specific antibodies (IgG, IgA) in milk against the SARS-CoV-2 virus before and after the mRNA vaccine.
Upon results, postvaccine (day 14) IgA antibody was positive in 60% of participants, and 100% of them had significant levels of IgG antibody in breastmilk. Furthermore, a spike-specific secretory antibody was shown in 50% of participants' breastmilk -BNT162b2 (Pfizer-BioNTech) and ChAdOx1 nCoV-19 (Oxford-AstraZeneca; ChAdOx1) vaccines are designed and developed by a Russian pharmaceutical company called "Gameleya" (Table 10). A genetic sequence for the full-length glycoprotein 'S' of the SARS-CoV-2 was recombined with adenovirus. e two vaccines were found to be more efficacious when they were mixed [41]. Volunteers receiving these vaccines showed no major adverse events, and their serological analysis revealed a 100% seroconversion rate and the presence of neutralizing antibodies on day 28. e analysis also indicated the presence of CD4+, CD8+, and IFN-c in all the vaccine recipients. ese antibodies demonstrate high efficacy against RBD of SARS-CoV-2 [42].
3.11. Ad26.Cov2.S Vaccine. Janssen Pharmaceutical has designed the vaccine based on the same principle that was used for the development of the Ebola vaccine (Table 11). e adenovirus vector Ad26 was used to carry the genetic sequence to the host cells. e company claims that the administration of a single dose of vaccine has produced neutralizing antibodies in 90% of vaccinated people after two weeks [43]. e vaccine in the clinical trials showed 66% of efficacy in protecting against the development of SARS-CoV-2 infection. e vaccine has also shown efficacy against the B, 1,351 variants of the virus [44]. Most of the study participants have shown no serious side effects and mild reactions are well tolerated. Pathological blood clotting is rarely seen in patients and is linked to low levels of platelets that trigger unexpected hypercoagulation [45]. One of the causes could be due to the wrong techniques of vaccine administration. If a vaccine enters the blood circulation in a large concentration, it may produce thrombocytopenia followed by hypercoagulability [46]. ere has been no confirmed report of any of the COVID-19 vaccines having a negative impact on male and female reproductive systems, though some concerns have been expressed in the published literature [50].

Third (Booster) Dose of COVID-19 Vaccine
It is the additional dose of the COVID-19 vaccine after the protection (antibodies) levels start to wane. According to the available reports, the level of antibodies against COVID-19 started to decrease from 4-6 weeks postvaccination. e data is not clear about the role of the B-cell that normally stores the memory for synthesizing the antibodies against the antigens [52]. However, considering the severity of infection, a booster/third dose is recommended for all those patients who have weak immunity. FDA has suggested the third dose of Pfizer/Moderna for cancer, organ transplant, stem cell transplant, HIV, and other such patients who are under high dose of immunosuppressants. ese patients were recommended to receive the booster dose after 28 days of the Out of 552 participants who received either covishield or COVAXIN, 79% of them were seropositive and responders for antispike antibodies. However, the covishield vaccine showed a significantly higher rate of respondence compared to COVAXIN.
Among 552 participants, Covishield vaccine showed a significantly higher incidence of adverse events compared to COVAXIN second dose [53]. Ideally, the same dose of the vaccine is recommended for the third dose, and more often it is done for those who took the mRNA vaccines. Studies in the past indicated that the administration of mRNA vaccines (Pfizer/Moderna) produced a weak immunological responses in patients suffering from immune system disorders [54]. Due to the appearance of mutant strains of SARS-CoV-2, clinical trials are also under progress to test the combined efficacy of COVID-19 vaccines. e pilot studies conducted after mixing the COVID-19 vaccines obtained from different sources have shown robust production of neutralizing antibodies in the test population [55]. One of the reasons reported is that the variation in vaccine technique might boost the immune system better without showing the tolerance towards the second dose of vaccine. However, there are reports indicating that such a combination may increase the complications. Important information about combinations of vaccines being trialed is represented in Table 12 [55][56][57][58][59][60][61][62][63].
Our observations from the review indicated that some of the COVID-19 vaccines have shown inflammatory reactions. e COVID-19 infection is associated with some risky inflammatory conditions such as vascular inflammation, myocarditis, and cardiac arrhythmias (Table 12). e binding of SARS-CoV-2 to ACE2 causes inflammation of the myocardium and lungs, causing injury to these organs [62]. One of the pathways for this is due to the release of several inflammatory mediators, partially because of ACE2 signaling. In previous studies, it was reported that the administration of vaccine for respiratory viruses such as influenza A and influenza B also produced inflammatory conditions. However, the effects of vaccination on the induction of inflammatory events in a few individuals need further research [63]. Furthermore, recent research has revealed that each vaccination has an almost similar degree of efficacy during clinical trials as well as when it is provided to the public (Table 13). Furthermore, multiple data (Table 14) shows that combining different vaccines during the second injection has no substantial detrimental impact.

Conclusion
COVID-19 vaccines have been safely administered to millions of people. All of the COVID-19 vaccines that have been approved have been thoroughly tested and are still being monitored. COVID-19 vaccines, like all vaccines, undergo a multistage testing process that includes large clinical trials involving thousands of individuals. ese tests are intended to uncover any potential safety issues. is review examined the key data reported during the COVID-19 vaccine clinical trials. Despite the fact that the vaccines were developed using different technologies, they demonstrated a nearly identical ability to produce strong neutralizing antibodies against SARS-CoV-2 during clinical trials and in real-world practice among different segments of society. All of the vaccines were well tolerated, with only minor side effects. A few serious complications, including thrombocytopenia, anaphylaxis, myocarditis, and Guillain-Barre syndrome, were rarely observed in postvaccination people, but the exact cause was unknown. e duration of the immunogenic response, efficacy of the mutants' SARS-CoV-2 strains, and precise reasons for the life-threatening complications could not be confirmed based on the trial's data and need more in-depth investigation. Studies are also essential for determining the efficacy of vaccine combinations as well as the need for booster doses in the management of COVID-19. Further studies are required to determine these vaccines' efficacy against COVID-19 mutants like omicron.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare no conflicts of interest.